KABUL, Sept 29 (Reuters) - A suicide bomber killed 28 Afghan troops and two civilians on Saturday in an attack on an army bus in Kabul, the Afghan president said.

The Taliban claimed responsibility for the attack, the deadliest in the Afghan capital since the hard-line Islamist movement was ousted from power for harboring al Qaeda leaders following the Sept. 11, 2001, attacks on the United States.

A suicide bomber dressed in army uniform got on the bus carrying Afghan National Army personnel to work, the Defense Ministry said. The blast split the bus into two and shattered shop windows in a central district of the capital.

Police and soldiers piled bodies onto army vehicles. Residents also helped pick up pieces of flesh and put them into plastic bags. A boy stood crying next to a police cordon saying he was looking for his father who sold cigarettes from a hand-cart next to the site of the bomb.

"The explosion happened just after a group of Afghan National Army soldiers got onto the bus," said witness Mohammad Zaher who had cuts on his forehead from flying glass.

"I heard a big bang. I saw the bus ... destroyed," said witness Ahmad Waleed. "I saw several bodies of military personnel being carried from the site alongside bodies of some workers who were sitting at the roundabout. The army and police arrived very late."

'AGAINST ISLAM'

President Hamid Karzai condemned the blast, which he said killed 28 soldiers and two civilians.

"It was an act of extreme cowardice on the part of those that committed it. No doubt someone who did this was against people, against humanity, definitely against Islam," Karzai told reporters at his heavily fortified palace.

"(It is) something we condemn in the strongest possible terms and something that would indicate to us that the war against terrorism must go on with much strong vigor," he said, speaking in English.

The U.N. Security Council also condemned the attack and in a statement reiterated concern about "the increasing threat to the local population, national security forces, international military and international assistance efforts."

One soldier from the International Security Assistance Force (ISAF) was killed in a clash with Taliban rebels in eastern Afghanistan on Saturday, a spokesman for the NATO-led force said.